An earthquake and tsunami triggered a loss of offsite power accident (LOOP), or “station blackout.” Onsite power was also lost, leading to core melt accidents and hydrogen explosions in units 1, 2, and 3.
Unit 4, which had no fuel in the reactor at the time, also suffered a large hydrogen explosion, which released radioactive material from the ‘spent’ fuel pool (SFP), and caused significant structural damage to the SFP.
All four units were GE Mark 1 Boiling Water Reactors – the same design as Pilgrim.
The Mark 1 containment was known to be defective at least as early as 1975, and three GE engineers resigned because their concerns weren’t taken seriously. Most Mark 1 reactors were later equipped with a vent system. This system has had three real-world tests (units 1-3) and all three failed completely. A containment with a vent is not a containment, it’s a sick joke.